In some wrap-packaging operations polymer films are desirably used which are self-sealing when portions are overlapped. Many self-sealing films (sometimes called "high-cling" films) have as much cling on one side as it does on the other, and this is often a detrimental feature. There are wrapping operations wherein a wrapping film is stretched tightly around an article or group of articles, and self-sealed in the stretched condition.
It has been known for many years that tackifiers can be added to a polymer to give the polymer the necessary tackiness or "cling" to adhere to itself, such as in U.S. Pat. No. 3,025,167, but such tackifiers, such as polybutene, terpene resin, hydrogenated rosin and rosin esters, create cling on both sides unless steps are taken to provide differential cling.
Polymeric films, e.g. polyolefin films, having one-sided cling properties are disclosed in U.S. Pat. 4,542,188. These involve the blending of a tackifier polymer, e.g. low molecular weight polybutene or atactic polypropylene (of less than 5000 molecular weight), with a "first" polymer and a "second" polymer, then extruding a film of the blended polymers at temperatures sufficient to maintain the blend as a liquid and cooling the extruded blend at rates sufficient to make a film having one side which has more cling than the opposite side. The patent discloses that preferably the "first" polymer is a low density polymer of ethylene, such as low density polyethylene (LDPE), and the "second" polymer is preferably a copolymer of ethylene and alpha-olefin of at least C.sub.3. The polybutene preferably has a molecular weight of less than about 2000.
The use of high molecular wt. ethylene/vinyl acetate copolymer (containing 4-25% by wt. of the VA) blended with a linear copolymer of ethylene/higher alkene as a stretch wrap film is disclosed in U.S. Pat. No. 4,425,268. The patent also discloses that cling properties can be added to the stretch wrap film by adding a low molecular weight polyisobutene, a polyterpene, amorphous polypropylene or microcrystalline wax as a tackifier. The patent further discloses that other additives besides the tackifier may be added, such as fatty acid monoesters or an alkali metal stearate.
U.S. Pat. No. 4,327,009 and U.S. Pat. No. 4,415,691 disclose the high cling of films produced from LLDPE having densities "in the low range of about 0.90-0.93 gm/cc" and high block. They disclose a method for reducing block, but retaining high cling.
EPO Application 0.287.272 discloses a plastic cling film which comprises, inter alia, at least one layer which is a LLDPE polymer exhibiting cling on at least one surface which is attributed to a relatively high inherent level of n-hexane extractable material in the polymer. We have not, however, found the level of inherent n-hexane extractables to be the correlative or determinative factor in cling properties, but have found density levels and density differentials to be the determinative factor.
Various methods have been disclosed which seek to provide differential cling properties whereby one side of a wrap film exhibits desirably strong cling properties while the other side of the film exhibits little or no cling properties. In this disclosure, we refer to these type of films as having "differential cling". In a general sense, the methods for providing films having such differential cling properties have included the use of:
(1) film laminates of at least two different polymer layers, including co-extruded films, wherein one side of the film laminate has the desired cling properties, the cling layer sometimes containing a tackifier;
(2) surface treatment (e.g., chemical, thermal, and/or electrical, such as corona discharge) of films whereby cling properties are imparted to one side of the film; and
(3) addition of tackifiers which are blended into the principle film polymer and the blend then extruded and cooled as a film in such a way as to cause the tackifiers to exhibit differential cling properties when applied as a wrap-packaging material.
It is the first method above, i.e. the addition of at least one tackifier to at least one of the polymers used in a film laminate structure, to which the present invention most closely pertains.
In this disclosure, the layer which comprises the outer "no-cling" layer of the laminate when applied as a packaging material is considered to be the "obverse" layer and the inner "high-cling" layer is the "reverse" layer. Ordinarily, it is the obverse layer which forms the exterior of a wrapped package or article, though there may be some instances wherein the high-cling "reverse" layer may be used as the outer layer, such as when there is some reason one would not want to use the high-cling layer in contact with the goods being wrapped. The "no cling" layer may actually exhibit a small amount of cling, according to the test procedure which is used in measuring the cling properties, but here it means that the amount of cling is insufficient to obtain a good self-seal when used in stretch-cling applications.
In this disclosure an "effective amount" of cling is that amount which is enough to provide the desired cling for the targeted application. It will be understood that some applications require more cling than other applications, and the present invention provides a means for regulating the amount of cling in the laminate by using pre-determined amounts of cling additives in the reverse layer, while holding the amount of cling in the obverse lager to significantly lower, acceptable levels or even to substantially no cling in the obverse layer.